The long-term goal of our research program is to understand the process of selective gene activation in molecular terms using Drosphila as a model system. The specific objectives of this proposal are to explore the question of reversibility in the mechanism of gene activation and to study in detail the chromatin structural alterations that occur on activation of a hormonally regulated gene. Recent studies in our lab and others have shown that active genes possess an altered chromatin structure; this change is not dependent on transcription per se. Certain nonhistone chromosomal proteins (different from RNA polymerase) are associated with both active genes and genes that will be active in the salivary gland cells. We plan experiments to see if we can detect alterations in chromatin structure preceding transcription which might be related to cell commitment and to determine whether or not such alterations are reversible. We also plan to map the regions of chromatin structure alteration in relation to the transcribed region of specific loci. These experiments involve assessment of the digestion of chromatin with DNase I and micrococcal nuclease using specific recombinant plasmid probes for a heat-shock locus and a hormonally regulated locus.